Ah; I missed that because we have the X11 checking stuff done in hwloc.m4 already -- I thought the ones in the Cairo were actually redundant.

It looks like this logic isn't quite correct, anyway -- the X11 checks are embedded in the Cairo and GL sections. Should they moved out to be independent of Cairo and GL (and therefore only once, and include the AC_DEFINE for HWLOC_HAVE_X11)?